The invention relates to hearing aids, and more particularly relates to hearing aids which can be used in noisy environments.
It has long been known to vary the characteristics of a hearing aid in accordance with the ambient noise level so as to make it easier for the patient to distinguish between useful information (i.e. speech) and noise. One system for accomplishing this objective varies the gain at low frequencies when the incoming low frequency energy (which is assumed to be mostly noise) exceeds some predetermined quantity. It has also been proposed to divide the incoming signal into a plurality of frequency bands and to adjust the audio gain for each band in dependance upon the signal-to-noise ratio in that band. Thus, where the signal-to-noise ratio in a particular band is poor, the gain in that band is cut back. Yet another approach involves the formation of a raw estimate of noise level, in which noise is estimated only during pauses between speech sounds.
One problem with these approaches is that they do not take full account of the known effects of noise on speech intelligibility. It has long been known that the effects of low-frequency noise are not restricted to low-frequency speech information. Rather, low-frequency noise also reduces the intelligibility of higher--frequency speech information. While this phenomenon--known as the upward spread of masking effect--is generally applicable to all human beings, it is more severe in hearing-impaired individuals.
It would be advantageous to produce a hearing aid signal processing system which took the phenomenon of spread of masking into account and corrected for it.